] 28 SEXUAL SELECTION : BTEDS. Part IL 



With respect to the slight individual differences which 

 are common, in a greater or less degree, to all the 

 members of the same species, we have every reason 

 to believe that they are by far the most important 

 for the work of selection. Secondary sexual characters 

 are eminently liable to vary, both w^ith animals in a 

 state of nature and under domestication.**^ There is 

 also reason to believe, as we have seen in our eighth 

 chapter, that variations are more apt to occur in the 

 male than in the female sex. All these continirencies 

 are highly favourable for sexual selection. Whether 

 characters thus acquired are transmitted to one sex 

 or to both sexes, depends exclusively in most cases, 

 as I hope to shew in the lollowing chapter, on the 

 form of inheritance which prevails in the groups in 

 question. 



It is sometimes difficult to form any opinion whether 

 certain sliirht differences between the sexes of birds 

 are simply the result of variability with sexually- 

 limited inheritance, without the aid of sexual selection, 

 or whether they have been augmented through this 

 latter process. I do not here refer to the innumerable 

 instances in which the male displays splendid colours 

 or other ornaments, of which the female partakes only 

 to a slight degree ; for these cases are almost certainly 

 due to characters primarily acquired by the male, 

 having been transferred, in a greater or less degree, to 

 the female. But what are we to conclude \^ith respect 

 to certain birds in which, for instance, the eyes differ 

 slightly in colour in the two sexes ?*^ In some cases 

 the eyes differ conspicuously; thus with the storks 



40 On these points see also ' Variation of Animals and Plants under 

 Domestication,' vol. i. p. 253 ; vol. ii. p. 73, 75. 



•*^ See, for instance, on the irides of a Podica and Gallicrex in ' Ibis,' 

 vol. ii. 1860, p. 206 ; and vol. v. 1863, p. 426. 



